The long-term objective of this research program is to understand the physiology of sympathetically dependent pain, such as that in reflex sympathetic dystrophy, acute herpes zoster and some cases of low back pain. The experiments proposed here focus upon trauma-induced changes in sympathetic activation of spinal neurons and deep-tissue afferents - trauma being a precipitating factor in many cases of sympathetically dependent pain. Aim 1. To determine the effects of trauma on sympathetic activation of deep tissue primary afferents. Single unit recording in anesthetized cats will be used to characterize deep-tissue afferents responsive to sympathetic stimulation; to quantify trauma-induced changes in the responses of these afferents as a function of time subsequent to trauma; and to examine interactive effects of trauma and tonic sympathetic activity on the response characteristics of the afferents. Aim 2. To determine the effects of deep-tissue trauma on sympathetically evoked activity in spinal neurons. Single unit recording from spinal neurons in anesthetized cats will be used to quantify traumainduced changes in the response characteristics of somatosensory neurons and to determine the combined effects of trauma and tonic sympathetic activity on the response properties of the neurons. Additional experiments will be done to differentiate central from peripheral effects induced by trauma on the responses of spinal neurons to sympathetic stimulation. Aim 3. To determine which neurotransmitters in the periphery are responsible for sympathetic activation of spinal neurons and deep-tissue afferents. Sympathetic neurotransmitter antagonists will be systemically applied during single unit recording from functionally identified primary afferents and spinal neurons. Alterations in the responses of the afferents and neurons to sympathetic stimulation will be quantified. Aim 4. To determine the topographical distributions of spinal neurons responsive to sympathetically-activated afferents from the hindlimb or to trauma-activated afferents from the low back. Immunocytochemistry will be used in anesthetized rats to label neurons that express c-fos. Expression will be induced by sympathetically evoked afferent activity after noxious conditioning stimulation in the hindlimb or by noxious stimulation of paraspinal tissues in the low back.